Feature: Skip serialization/deserialization of a specific field.

Feature: Use your own serialization/deserialization routines for custom class and struct types.

msgpack-d provides the functions registerPackHandler / registerUnpackHandler to allow you
to use custom routines during the serialization or deserialization of user-defined class and struct types.
This feature is especially useful when serializing a derived class object when that object is statically
typed as a base class object.